The platelet factor 3 assay and circulating immune complexes as studied on non-thrombocytopenic patients with...

Sera from 21 patients with systemic lupus erythematosus (SLE) were tested with a platelet factor 3 (PF3) assay. A control serum (pooled blood group AB serum) was always run in parallel with the test sera. The test and control sera were run in duplicate and for each patient the mean recalcification time was used for the calculation of the test serum: AB serum ratio. Concomitantly, the patients' serum concentrations of circulating immune complexes (CIC) were measured by using the Raji cell radioimmunoassay. 20 healthy hospital employees served as controls. In the latter subjects the mean test serum: AB serum ratio was 0.99 (range 0.92-1.09), their CIC concentrations being less than or equal to 16 micrograms/ml. 7 of the SLE patients had CIC values less than or equal to 16 micrograms/ml, and their test serum: AB serum ratios ranged 0.95-1.09. All the remaining SLE patients had elevated values for CIC. In the total material of SLE there was a highly significant (r = -0.87; P less than 0.001) negative correlation between the test serum: AB serum ratios and the values for CIC. It is widely held that the so-called PF3 immunoinjury test detects circulating antiplatelet antibodies. The present results, however, strongly suggest that the assay rather or also detects CIC which like antiplatelet antibodies may damage platelets as we and others have shown in prior publications.